1. Technical Field of the Invention
This invention relates to control systems and methods for controlling inverter based electrical power generation and feeding of generated power to a grid. This invention particularly relates to an integrated control system and method that integrates a variety of power control functions including state machine control of distinct operational modes, synronization with the grid, power factor control and utility outage ride-through.
2. Description of Related Art
Various control devices for controlling inverter based electrical power generation are known in the art. Typical controllers utilize analog voltage or current reference signals, synchronized with the grid to control the generated wave form being fed to the grid. Such controllers, however, lack distinct control states and the capability of controlling transitions between specifically defined control states.
Various techniques for synchronizing the frequency of generated power to the frequency of a grid-are also known in the art. Such conventional line synchronizers typically sense the line frequency of the grid and lock to the grid when the generated frequency drifts into synchronization.
Such conventional line synchronizers, however, do not have the ability to control the rate of phase shift of the generated power or the ability to interface easily with both 50 Hz and 60 Hz grids.
Various techniques for controlling the power factor are also known in the art. In the context of electrical power generation, for example, Erdman, U.S. Pat. No. 5,225,712, issued Jul. 6, 1993, discloses a variable wind speed turbine electrical power generator having power factor control. The inverter can control reactive power output as a power factor angle or directly as a number of VARs independent of the real power. To control the reactive power, Erdman utilizes a voltage waveform as a reference to form a current control waveform for each output phase. The current control waveform for each phase is applied to a current regulator which regulates the drive current that controls the currents for each phase of the inverter.
Although the conventional art may individually provide some of these features, the combination of these features particularly when utilized in conjunction with an integrated system utilizing state machine control is not found in the art.
Other applications distinct from electrical power generation also utilize power factor control devices. For example, Hall, U.S. Pat. No. 5,773,955 issued Jun. 30, 1998, discloses a battery charger apparatus that controls the power factor by vector control techniques. The control loop utilized by Hall controls power delivery to the battery to obtain a desired charge profile by individually controlling the real and reactive components of the AC input current. The AC input current is forced to follow a reference that is generated in response to information received by the battery charge control circuit to supply the desired charging current to and remove discharge current from a battery.
An object of the invention is to provide an integrated system for controlling all aspects of inverter based electrical power generation and feeding of generated power to a grid. Another object of the invention is to provide a state machine having a plurality of defined control states for electric power transformation including a state controller that controls permitted transitions between the defined control states.
Another object of the invention is to provide a line synchronization technique that is highly flexible and permits synchronization with either a 50 Hz or 60 Hz grid as well as providing smooth transitioning from a stand-alone mode to a grid-connected mode.
A further object of the invention is to provide a line synchronization technique that can either sense the grid frequency or synthesize a frequency for electrical power generation.
Still another object of the invention is to control the re-synchronization rate to provide the smooth transition from stand-alone mode to a grid-connected mode.
A further object of the invention is to provide a method of controlling an electrical power generator during a utility outage.
Yet another object of the invention is to integrate the inventive method of utility outage ride-through with various other control techniques to provide an integrated system.
Still another object of the invention is to provide power factor control over generated electrical power wherein a simple DC control signal having two components commanding the real and reactive components of the generated power may be utilized to control the power factor.
The objects of the invention are achieved by providing a state machine having a plurality of control states for electric power transformation including an initialization state, a first neutral state, a pre-charge state, a second neutral state, an engine start state, a power on-line state, a power off-line state, and a shut down state wherein the state controller controls state transitions such that only permitted transitions between control states are allowed to occur. In this way, a high degree of control can be achieved for electrical power generating and feeding of electrical power to a grid. In this way, the safety and reliability of the system can be ensured.
The objects of the invention are further achieved by a method of controlling real and reactive power developed by a main inverter in an electrical power generation control device including the steps of sampling the three-phase currents output from the inverter, transforming the sampled three-phase current data to two-phase current data, transforming the two-phase current data to a rotating reference frame, controlling an output voltage according to a comparison result between a DC reference signal having real and reactive reference signal components, transforming the output voltage to a stationary reference frame, transforming the stationary reference frame output voltage to a three-phase reference signal, and controlling the inverter based on the three-phase reference signal. By utilizing such a control method, the DC reference signal can be input by an operator or a utility feeding the grid to thereby designate the real and reactive power output by the controlled inverter.
The objects of the invention are further achieved by providing a line frequency synchronization apparatus and method that utilizes a frequency sensor that samples the frequency of the grid or a synthesizer that synthesizes a grid frequency. In the case of sampled grid frequency, the frequency sensor signal is converted by an A/D converter that is controlled by initiating the conversion and reading of the digital value at a fixed frequency. This fixed frequency establishes the time base for which the invention can compute the actual frequency of the signal. This is further accomplished by determining when the falling or rising edge of the signal occurs and counting the number of samples therebetween.
In this way, a synchronization error signal is generated that can be utilized to bring the generated power into synchronization with a grid or the synthesized grid frequency. Furthermore, the synchronization shift rate is preferably limited in order to provide a smooth transition.
The objects of the invention are further achieved by providing a utility outage ride-through method and apparatus that detects a fault condition indicating that the electrical power generation device should be disconnected from the grid, opens a contactor that connects the device to the grid, clears a time counter, sets a mode to an off-line mode, commands the inverter within the device to perform off-line voltage control, and waits for a predetermined time period after all fault conditions have been cleared before setting the mode to an on-line current control mode, enabling the inverter and thereafter closing the contactor to reestablish the connection to the grid.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.